In recent years, as the demand for optical communication network has increased, system lines in a bureau or premises are required to contain a larger number of core cords therein. In order to allow lines to contain a larger number of core cords therein, a wide space has to be allocated for lines. However, as the space for lines in a bureau or premises is limited, making the respective diameters of optical fiber cords smaller is essential. It should be noted, however, that when the diameter of each optical fiber cord is made smaller, the optical fiber cords still need to be held in a loose state, so that a core wire does not buckle when the core wire is pushed into the optical fiber cord at the time of attaching a connector; and the mechanical properties, such as the tensile strength and bending rigidity of the optical fiber cord, still need to be kept at a predetermined level or higher, so that excellent handling properties are maintained in jumper ring, such as line-switching. In addition, as the optical fiber cords are used indoors, they must have an excellent fire-retardant property. Due to this, polyvinyl chloride (PVC) has conventionally been used as a coating material thereof.
As examples of efforts to make the diameter of optical fiber cord smaller, there are JP-A-10-10380 (“JP-A” means unexamined published Japanese patent application), JP-A-2000-28875, and the like. These examples are characterized in that the diameter of conventional optical fiber cord is made smaller.
On the other hand, in recent years, there has been a problem that, when a coating material containing polyvinyl chloride or a halogen-series fire-retardant agent is discarded without being properly treated, for example, a plasticizer and/or a heavy metal stabilizer, which have been blended in the coating material, dissolve out. In addition, another problem, that a large quantity of smoke and hazardous/corrosive gases are generated when such a coating material is burned, has been an issue. In particular, it has recently been reported that the coating material may be a source of dioxin.
In consideration of such an influence on the environment by a coating material containing PVC or a halogen-series fire-retardant agent, non-halogen fire-retardant coating material, in which metal hydrates are filled at a high concentration in a polyolefine-series resin component, has been studied, in place of the coating material containing halogens, such as polyvinyl chloride. As one example in which a non-halogen fire-retardant coating material is used for a coating material of an optical fiber core wire or an optical cord, there are JP-A-9-33770 etc. However, this conventional example is characterized in that it only attempts to make the conventional optical fiber cord fire-retardant by a non-halogen material, and it gave no consideration to other matters, including making the diameter of the optical fiber cord smaller.
When an optical fiber cord is coated with a composition in which metal hydroxides are filled at a high concentration, if the diameter of the optical fiber cord is made smaller, a base resin having relatively low elastic modulus must be used as the base resin of the coating material, so that the fire-retardant agent is blended at a high concentration and in an excellently dispersed manner. Accordingly, it has been difficult to obtain mechanical properties, such as bending rigidity, that are necessary for an optical fiber cord. In particular, in the case of the outer diameter of the optical fiber cord is 1.2 mm or less, when a single layer of a composition in which metal hydroxide is filled at a high concentration is used as the coating of the optical fiber cord, bending rigidity of a given level necessary for the optical fiber cord may not be obtained, or a problem that, when the optical fiber cord is held for a long time so as to have a constant bending diameter, the optical fiber cord tends to remain in a bent form even after the cord is released, may arise. Further, when jumper ring (switching of optical lines) of optical fiber cords is performed, an operation for pulling out desired optical fiber cord terminals from a bundle of densely lined optical fiber cords is necessitated. In order to avoid the occurrence of buckling of the cords during the operation, bending rigidity of a certain value or higher is necessary.
The present invention has been contrived to solve the aforementioned problems. An object of the present invention is to provide an optical fiber cord having a diameter that has been made smaller, to 1.2 mm or less, and having excellent fire-retardant, mechanical, and handling properties.
Other and further objects, features, and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.